Orientation

Astronomy would be easier but less interesting if the Earth stood
still. In fact, the Earth has several different motions which
astronomers long ago worked hard to understand. One of these motions
is the Earth's rotation on its axis; another is the
Earth's revolution, or orbital motion, around the
Sun.

ROTATION

If you watch the night sky for a few hours, you will see that the
stars appear to rotate about a
fixed point in the sky, known as the north celestial
pole, which just happens to be near the star Polaris. This is
due to the Earth's rotation. As the spinning Earth carries us
eastward at almost one thousand miles per hour, we see stars rising in
the east, passing overhead, and setting in the west. The Sun, Moon,
and planets move across the sky much like the stars.

Ancient astronomers explained this by supposing that the Sun, Moon,
planets, and stars were all attached to a huge celestial
sphere, centered on the Earth, which rotated on a fixed axis
once per day. Of course, this sphere does not really exist; the Sun,
Moon, planets, and stars all fall freely through space, and
only appear to move together because of the Earth's rotation.
Nonetheless, we still use the concept of the celestial sphere when
talking about the positions of stars.

The celestial pole is 21.3° above the horizon as seen from
Oahu. The point on the horizon directly below the celestial pole is
north, while the opposite direction is
south. If you face north, east is on your
right and west is on your left. Finally, the
zenith is the point exactly overhead.

The north celestial pole is exactly overhead the Earth's north
pole. Likewise, every point on the celestial equator is
exactly overhead a corresponding point on the Earth's equator.

REVOLUTION

Once a year, the Earth makes a complete orbit about the Sun. As a
result, the Sun appears to move
with respect to the stars, passing in front of one constellation
after another, as shown in the diagram on p. 12 of Stars &
Planets. After one year, the Sun is back where it started. The
Sun's annual path across the sky is called the ecliptic.
Traditionally, the ecliptic was divided into twelve equal parts, each
associated with a different constellation. The planets also appear to
move along the ecliptic, although, as we will see, they don't always
move in the same direction as the Sun.

The night sky is just that part of the sky which we see when the
islands of Hawaii have turned away from the Sun. As we orbit the Sun,
different constellations are visible at different times of the year.
In September, for example, the evening sky is still dominated by
summer constellations like Cygnus and Sagittarius; by December, these
constellations will be low in the western sky, and winter
constellations like Taurus and Orion will be rising in the east. You
can get a `sneak preview' of the winter sky by staying up late, thanks
to the Earth's rotation. For example, the constellations visible at
8 pm in early December can also be seen at 2 am in early
September.

The Earth's axis of rotation is not parallel to its axis of
revolution; the angle between them is 23.5°. As a result, the
ecliptic is tilted by the same angle of 23.5° with respect to the
celestial equator. This misalignment causes seasons;
when the Sun appears north of the celestial equator the Earth's
northern hemisphere receives more sunlight, while when the Sun appears
south of the celestial equator the northern hemisphere receives less
sunlight.

If we could view the Solar System from a point far above the north
pole, we'd see the Earth revolving counter-clockwise about the Sun and
rotating counter-clockwise on its axis. The other planets would
likelwise revolve counter-clockwise around the Sun, and most would
also rotate counter-clockwise. In addition, the Moon would appear to
orbit the Earth in a counter-clockwise direction, as would most other
planetary satellites.

TIME

In this class, we will use a 24-hour clock instead of writing `am'
or `pm'. Since our class meets in the evening, most of the times we
will record are after noon, and the 24-hour time is the time on your
watch plus 12 hours. In the notes, this will be called `Hawaii
Standard Time' and indicated by writing HST. For
example, our class starts at 19:00 HST (= 7:00 pm +
12:00), and ends at 22:00 HST (= 10:00 pm + 12:00).
Sometimes we need to record the date and the time together; for
example, our first class began at 25-Aug-08 at 19:00
HST.

Astronomers all over world use a single time system to coordinate
their observations. This system is called Universal
Time, abbreviated as UT or UTC
(Greenwich Mean Time, abbreviated GMT, is the same
thing as UT). Universal Time is exactly 10 hours ahead
of Hawaii Time. To convert 24-hour Hawaii Time to UT,
you add 10 hours; if the result is more than 24, subtract 24 and go to
the next day. For example, our first observing session (weather
permitting) will be at 08-Sep-08 at 19:00 HST, or
09-Sep-08 at 05:00 UT. To convert from
UT to Hawaii time, subtract 10 hours; if the result is
less than 0, add 24 and go to the previous day. For example, we can
see a stellar occultation (the eclipse
of a star by the Moon) on 07-Oct-08 at 05:05 UT; that's
06-Oct-08 at 19:05 HST, just after the start of class.

ALL-SKY CHARTS

Astronomers represent the appearance of the entire sky as seen at
some particular place and time by drawing circular all-sky
charts. Unfortunately, it's hard to show how the sky really
looks using a flat piece of paper, so reading an all-sky chart and
relating it to what you see in the sky takes practice. For example,
these charts distort the patterns of stars near the horizon, so you
may find it hard to recognize constellations from an all-sky chart.
The only way to correct this distortion is to break the sky up into
several separate charts. For some purposes, however, it's very
convenient to show the entire sky in one chart, so you should learn to
read these charts. All-sky charts for each month appear in Stars
& Planets, starting on p. 24.

To read an all-sky chart, hold it in front of you with the side
labeled `N' at the top. Now imagine you are lying flat on your
back with your head pointing north; then east will be on your left,
south at your feet, west on your right, and the zenith right in front
of you. In your mind, stretch the chart so that it forms a dome over
your head. The positions of stars on this imaginary dome now match
their positions in the sky.

You can get a pretty good idea of how the sky will look on
09-Sep-08 at 20:30 HST by using the chart shown in
Fig. 1. For example, the constellation of Cygnus (the swan),
which is outlined in red, appears near the center of the chart, so it
will be roughly overhead. Next to Cygnus is the bright star Vega; it
is very near but just slightly above the center of the chart, so it
will be seen very near the zenith but just slightly to the north. The
planet Jupiter (indicated by the largest dot) and the Moon appear near
each other, about half-way between the center of the chart and the
southern compass point; they will appear in front of you, abouf
half-way between the zenith and the horizon, if you face south. The
bright star Acrturus, known to Polynesian navigators as Hokule'a
(`star of gladness'), is seen toward the west, having passed almost
directly overhead a few hours earlier.

If you are used to reading maps of the Earth, you may notice that
the east and west compass points in Fig. 1 are reversed. On a
terrestrial map with north at the top, you would expect to find west
to the left and east to the right. However, a celestial map with
north at the top has west at the right and east at the left. The
reason for this is that a terrestrial map shows a view looking
down at the Earth, while a celestial map shows a view looking
up at the sky. Astronomical charts usually have north at the
top and west to the right. When using a telescope, you'll notice that
stars drift toward the west as a result of the Earth's rotation; this
makes it easy to determine the correct orientation of a star
chart.

Fig. 1. The sky over Honolulu on
08-Sep-08 at 20:30 HST (09-Sep-08 at 06:30 UT), produced using Stellarium. Stars are
shown as dots, with larger dots for brighter stars; the lines
between stars trace constellations, with the constellation Cygnus
highlighted in red. The blue curve is the celestial equator, and
the red curve is the ecliptic. Compass points are shown around the
edge of the chart.

WEB RESOURCES

An interactive planetarium, set up to show the sky now
above Honolulu. You can chose other dates and times, select
other viewing sites, and zoom in on selected areas; for these
and other options, see http://www.fourmilab.to/yoursky.
Created by John
Walker.

Shows how the sky above Honolulu at 20:30 HT
changes during one year, from 21-Dec-07 to 21-Dec-08. This
animation illustrates the effect of the Earth's revolution
around the Sun. Note how the constellations visible in the
night sky change as the Earth revolves around the Sun. Also,
note the Moon's monthly passages across the sky along the
ecliptic and the fairly gradual motion of other planets.

REVIEW QUESTIONS

If you face north, which way does the celestial sphere appear
to rotate - clockwise or counter-clockwise?

If you see the Moon rising in the east at sunset, where would
you expect to see it just before sunrise next morning?

What is 20-Sep-08 at 7:35 UT in local 24-hour
time? What day of the week? Is it morning or evening?

If you know what the constellation `The Big Dipper' looks
like, find it in Fig. 1. Which compass direction should you
look toward to see it? Is it rising or setting?